1. Field of the Invention
This invention relates to power transmission belt manufacture and more particularly to a method of making an endless power transmission ribbed belt. Such a ribbed belt is thin and flexible compared with ordinary V-belts, and is excellent in high-speed rotation and its ability to transmit power. Such ribbed belts are, therefore, widely used in drive devices of automobiles wherein the power transmission belt is arranged in a serpentine path about the crank pulley, alternator pulley, a cooler pulley, etc.
2. Description of the Background Art
A method (1) of forming a belt using a tubular matrix is disclosed in U.S. Pat. Nos. 3,839,116 and 3,981,206. The method disclosed therein comprises building up successively a lower fabric, a lower rubber layer, a tensile member, an upper rubber layer, and an upper fabric, on a tubular matrix having a plurality of V-grooves in a circumferential direction inserted into a cylindrical drum, to form a vulcanized belt sleeve having a plurality of V-grooves. The belt sleeve is then cut into individual belts.
Another method (2) of forming a belt using a grinding wheel is disclosed in U.S. Pat. Nos. 3,818,576, 3,822,516, and 4,177,688, in which the vulcanized belt sleeve is formed on a mandrel and ground with a contoured grinding wheel to precisely define the desired plurality of longitudinally extending ribs.
Another method (3) of forming a belt is using a Rotocure mechanism as disclosed in U.S. Pat. No. 4,139,406, in which a formed belt sleeve is laid over the Rotocure mechanism having a plurality of V-grooves to define the desired ribbed section belt.
A casting method (4) of forming a belt is disclosed in U.S. Pat. No. 3,813,197 and Great Britain Pat. No. 1,123,225, which comprises the fixing of an inner mold having a fabric and a tensile member wound thereon, in an outer mold having a plurality of V-grooves arranged circumferentially along its inner surface, casting urethane elastomeric material in the mold cavity to form a belt sleeve, and cutting the belt sleeve into the desired final belts.
In carrying out methods (1), (2) and (3), the method using a grinding wheel (2) and the method using Rotocure (3), the ribbed belts may be made of an ordinary rubber, such as NR, SBR, CR, etc. In method (1), the use of a flat lower rubber layer on the tubular matrix is disadvantageous in that the tensile members may fall into the V-grooves of the tubular matrix during vulcanization and thus become arranged randomly, which may result in an undesirable stretchable belt. However, the use of a ribbed lower rubber layer prevents such defects, and the manufacture of the belts is facilitated and improved. Another serious disadvantage in method (1) lies in the difficulty of removing the formed belt sleeve from the tubular matrix which conventionally causes a short lift of the tubular matrix.
In method (2), the surface temperature of the vulcanized belt sleeve reaches more than 100.degree. C. during the grinding process so that reversion, due to a break in the intermolecular bonds, occurs on the surface of the sleeve. This may produce troublesome sticky adhesion. Also, since the yieldable belt sleeve is deformed by the grinding wheel during the grinding operation, it is difficult to grind the V-grooves accurately. The material and roughness of the grinding wheel must be correlated with the hardness of the belt sleeve.
A problem arises in the method (3) manufacture in that the belt sleeve shrinks when it is removed from the heated Rotocure mechanism. The thermal efficiency of the process is therefore very low because the Rotocure mechanism must be cooled before the belt sleeve is removed necessitating repeating heating and cooling steps.
The above mentioned methods, which have been developed for making ribbed belts, have drawbacks concerning the accuracy of the V-grooves, sticky adhesion of rubber in proportion to the generation of heat, dimensional instability after vulcanization, high cost, etc.